Kaonashia insperata gen. et sp. nov., a eukaryotrophic flagellate, represents a novel major lineage of heterotrophic stramenopiles

Eukaryotrophic protists are ecologically significant and possess characteristics key to understanding the evolution of eukaryotes; however, they remain poorly studied, due partly to the complexities of maintaining predator–prey cultures. Kaonashia insperata, gen. nov., et sp. nov., is a free-swimming biflagellated eukaryotroph with a conspicuous ventral groove, a trait observed in distantly related lineages across eukaryote diversity. Di-eukaryotic (predator–prey) cultures of K. insperata with three marine algae (Isochrysis galbana, Guillardia theta, and Phaeodactylum tricornutum) were established by single-cell isolation. Growth trials showed that the studied K. insperata clone grew particularly well on G. theta, reaching a peak abundance of 1.0 × 10⁵ ± 4.0 × 10⁴ cells ml⁻¹. Small-subunit ribosomal DNA phylogenies infer that K. insperata is a stramenopile with moderate support; however, it does not fall within any well-defined phylogenetic group, including environmental sequence clades (e.g. MASTs), and its specific placement remains unresolved. Electron microscopy shows traits consistent with stramenopile affinity, including mastigonemes on the anterior flagellum and tubular mitochondrial cristae. Kaonashia insperata may represent a novel major lineage within stramenopiles, and be important for understanding the evolutionary history of the group. While heterotrophic stramenopile flagellates are considered to be predominantly bacterivorous, eukaryotrophy may be relatively widespread amongst this assemblage.